This invention relates to circuit detecting apparatus for detecting open input circuits in multipoint recording systems having a plurality of input circuits which are sequentially switched to connect to a servo-operated measuring circuit through a low pass input filter.
Heretofore a wide variety of open-circuit detecting methods have been utilized to detect open circuits in the inputs of various recording devices. They have been used particularly when the input circuits include thermocouples for temperature measuring purposes. The open-circuit detecting methods previously used have been of the type which required either an excessive number of components and/or an excessive amount of time to make the checking operation. In addition, prior art testing circuits frequently include testing methods which tend to saturate the amplifier of the servo control for the measuring circuit during the time between points when the input circuits are open. The effect of such saturation, of course, is adverse to any system in which rapid checking of consecutive points is required.
The novel apparatus of this invention is not of the type which requires a shorting or disconnecting of the input filter in order to prevent such problems. Likewise, parallel devices are not inadvertently affected by the check signal as is the case with some checking methods and similarly, noise within the instrument will not disturb the measurement or cause spiking in the apparatus of this invention.